The Truth About 4 Common Forensics Methods

In our feature story about forensics, PM gives an in-depth report on the shaky science that has put innocent people behind bars. Here, we take a close look at four common evidence-gathering techniques, and the debates behind their accuracy.

Fingerprints

Fingerprint evidence is captured from surfaces using powders, lasers, even Super Glue.

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The Technique

Fingerprint investigations strive to match a print left at a crime scene to a print on file or in a database. The accepted technique for scrutiny is called ACE-V, for analysis, comparison, evaluation and verification. Examiners study features such as ridge shapes, patterns and lengths, looking for agreement—some jurisdictions look for a certain number of "points" of similarity to constitute a match. The final step, verification, occurs when a second examiner reaches the same conclusion. Many law enforcement agencies use computerized fingerprint identification systems to identify potential matches, but human examiners always have the final say.

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The Debate

No studies have proved definitively that fingerprints are unique. Likewise, it is unclear if prints change over time or vary depending on the amount of pressure applied. Statistical models and pattern-recognition software could go a long way toward answering these questions. Additionally, research is needed to expose error rates. The ACE-V method depends on two or more examiners reaching the same subjective conclusion, but doesn't require them to reach it the same way. In one recent experiment, veteran examiners looking twice at the same print came to different conclusions each time.

Ballistics

Forensic examiners use the marks left on bullets to match them to specific firearms, but the technique lacks a solid base of research, and errors are common.

New 3D imaging technology can analyze bullet markings for both surface pattern and depth.

The Technique

The theory behind ballistics analysis is that the manufacture and use of a firearm produces unique tool marks inside the barrel, which are then transferred to each bullet fired from that gun. Forensic examiners measure bullet size to determine caliber, then check the direction of rifling marks and the degree of twist to narrow down the gun's manufacturer. To match a specific firearm to a bullet, investigators test-fire the weapon with a new slug and compare both bullets under a microscope, looking for identical striations. Investigators can also query computer databases that suggest potential matches.

The Debate

As with fingerprints, not enough research has been done to quantify the probability of error in ballistics matching. So it's impossible to say with certainty that the marks made on bullets as they are fired are truly unique to an individual gun. Currently, ballistics examiners are aided by computer databases such as the ATF's National Integrated Ballistic Information Network, but lab techs always rely on their own visual inspection to make the final call. The Association of Firearm and Tool Mark Examiners only requires an examiner to find "sufficient agreement" between bullets in order to conclude that they came from the same gun. Those judgment calls can cause false results. Last September the Detroit Police Department's crime lab was shut down after an audit by the state of Michigan found a 10 percent error rate in ballistics identification.

Trace Evidence

Analysis of paint, fibers and soil found at crime scenes has the potential to be good science, but more substantiating research is needed.

The FBI's National Automotive Paint File has more than 40,000 entries dating back to the 1930s.

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The Technique

Trace evidence found at crime scenes can include everything from paint residue to clothing fibers to deposits of soil. These samples are then subject to pattern-matching and microscopic and chemical analysis. In the case of paint, it's critical for the evidence-collection team to return a sample that includes all possible layers—this makes it easier to match a sample to the object it came from. If multiple paint chips are found from the same source, it is sometimes possible to piece the chips together by matching brush-stroke striations or abrasions. When comparing fibers that appear to be similar, examiners perform a series of analytical chemistry tests to determine composition (natural or man-made, for example), color variations, shape and solubility.

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Paint analysis has a relatively strong scientific backing, and it can provide reliable results. Studies have shown that more than 97 percent of random auto paint samples and 99 percent of architectural paint samples can be differentiated. That can be useful for establishing that two samples might have a common origin (say, a particular brand of car paint), but current techniques aren't sophisticated enough to rule out all other sources. Likewise, fiber analysis has a foundation in chemistry; however, more research is needed to determine the criteria for a match. Current methodology is only sufficient to conclude that fibers could have come from the same type of garment or carpet.

Biological Evidence

The research and rigor behind DNA science has turned biological evidence into the strongest tool in the courtroom.

Left, DNA must be extracted from white blood cells. Red cells lack nuclei. Right, DNA can be found in hair years after a crime has been committed.

The Technique

A critical challenge in utilizing biological evidence is first finding it, and then figuring out exactly what it is. Blood at a crime scene may not always be visible (if, for instance, someone has attempted a cleanup), so investigators apply chemicals, such as luminol, that change color as red blood cells release oxygen. Immunological tests can be performed in the lab to identify proteins, such as hemoglobin, that can indicate if the blood is human. Hair samples are examined microscopically for characteristics such as color, chemical treatment, shaft form (wavy, straight, curly) and pigment distribution. All biological samples have the potential to yield useful DNA evidence. When labs perform nuclear DNA testing, examiners look for matches on up to 16 short tandem repeat (STR) loci, which vary considerably from person to person. Where nuclear DNA is either degraded or not available—for instance, bones, teeth and hair samples that have no roots—the less-exacting mitochondrial DNA (mtDNA) testing process is typically employed.

The Debate

Before DNA testing, blood and hair were considered "class evidence." That means serological typing of blood samples and microscopic analysis of hair could only narrow down a list of possible suspects. Research has shown that matching hairs using subjective analysis can be highly inaccurate (one FBI study found a 12.5 percent error rate). By comparison, the statistical probability of a false positive using the most advanced DNA testing kits can be as low as one in more than a quadrillion.